Not that the gay community needs much more convincing that homosexuality is tied to biology, but new research finds a correlation between men being gay and their having older brothers. From exploring the lifestyles and biology of of nearly 1,000 Canadian men, the “fraternal birth order effect” study author Anthony Bogaert concludes there’s a link between the sexuality of a man and whether he’s got older brothers — even if he wasn’t raised with them.

Researchers have known for years that a man’s likelihood of being gay rises with the number of older biological brothers. But the new study found that the so-called “fraternal birth order effect” persists even if gay men were raised away from their biological families. […]

Bogaert and a colleague first reported the older-brother effect a decade ago. According to Bogaert, men with no older brothers have about a 2 percent to 3 percent chance of being gay. If they have three or four older brothers, the rate goes up to about 5 percent. […]

To find the answer, Bogaert examined surveys of 944 Canadian men, both gay and straight, about their sexuality and their families.

The older-brother effect was constant regardless of whether the men were raised with natural, adopted or stepbrothers. It also didn’t matter if they weren’t raised with their biological mothers.